This invention concerns primarily a method for nucleating ethylene terephthalate polymers. The nucleating agents are alkali metal salts of low molecular weight polyethylene terephthalate (hereafter: PET).
PET is used in very large volume in the manufacture of molded plastics, films, and fibers. A well-known problem in connection with use of PET is that it crystallizes relatively slowly under molding conditions compared with other polymeric materials. The rate of crystallization of PET can be increased, that is, the crystallization temperature in the melt can be raised, by incorporating into the molding composition any of a number of materials known as nucleating agents.
By and large, nucleating agents of the prior art that provide significant increases in crystallization temperature also cause undesirable reductions of molecular weight, whereas nucleating agents that do not significantly lower molecular weight tend to have little effect on crystallization behavior. The following publications are representative of the state of this art.
EPA No. 31,201 discloses in Example 2 the reaction of terephthalic acid with ethylene glycol in the presence of sodium hydroxide. This publication is believed to teach the making of a sodium salt nucleating agent in situ.
EPA No. 21,648 discloses reaction of PET with an ionizable metal salt of an organic compound having at least one acidic proton. The ionizable metal salt comprises sodium salts of substituted benzoic acids containing at least one substituent such as nitro, halo, hydroxy, phenyl, or oxyphenyl. In determining the carboxylate end-group content of the nucleated PET, PET polymer samples containing known amounts of --COO.sup..crclbar. Na.sup..sym. groups were used as standards. There is no disclosure of using these latter materials as nucleating agents.
EPA No. 25,573 discloses a fast-crystallizing polyester composition containing PET copolymerized and/or mixed with a polyoxyalkylene compound containing at least one --COOM group. The compounds can have the structure ##STR1## There is no disclosure of such compounds as nucleating agents.
U.S. Pat. No. 3,761,450 discloses lithium and/or sodium salts of aliphatic, cycloaliphatic, aromatic or heterocyclic polycarboxylic acids containing up to 20 carbon atoms as nucleating agents for PET.
GB No. 2,015,014A discloses sodium and potassium salts of selected organic polymers containing pendant carboxyl groups as agents for increasing the rate of crystallization of PET in composite materials.
U.S. Pat. No. 3,516,957 discloses alkali metal salts and alkaline earth metal salts of organic acids having 10 to 25 carbon atoms, e.g., sodium stearate, as nucleating agents for PET.
Japanese Patent application No. 46-29977 discloses sodium benzoate as a nucleating agent for PET.
U.S. Pat. No. 4,212,791 discloses compositions comprising PET, an inert, inorganic, particulate nucleating agent, an oligomer polyester crystallization promoter that does not contain carboxylate moieties, and a segmented thermoplastic copolyester-ether elastomer.